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44 SMT Magazine • July 2016 under test can be connected by the common technique of using connectors, since these are off the platen. In future PCB designs the tabs to the connectors will be extended away from the PCB area in contact with the platen to mini- mize further any tendency to cause condensa- tion on the connectors themselves. In Figure 2 the ability of the system to control the test board temperature is shown. The temperature profiles in Figure 2 include the chamber temperature, and it can be clearly seen that this does not change as the platen and PCBs cycle up and down by 2°C, with a nom- inal chamber temperature of 40°C. This is an important advantage of this approach in that there is no attempt to use transitioning in the humidity chamber condition to create tempo- rary condensing conditions. Furthermore, it can be seen that the transition is rapid in the cooling and heating phase of the cycle, and ad- ditionally the low temperature part of the cycle can be sustained indefinitely, or as long as set in the program cycle. The effect on surface insulation resistance (SIR) on a test PCB with 4 SIR patterns with a 400 µm track and 200µm gap in an ambient condition of 40°C/85% relative humidity (RH) is now explored. This PCB is not conformally coated, the ENIG finished copper is fully ex- posed. In Figure 3, the effect of cycling to in- creasingly lower temperatures is presented. Five sets of data are shown, in which the minimum temperature diminishes by 0.5°C from set to set. In each set the temperature re- turns to the ambient 40°C. These results show that it is not until the bottom temperature of the cycle reaches 37°C does the reduction in SIR become particularly significant. For the 1.5 to 2.5°C depression of the PCB temperature the level of condensation is only having a minimal effect, but for 3.0 and 3.5°C the condensation is clearly having a significant impact. From this it is clear that the 0.5°C steps provide a fine level of control and that a range of condensing con- ditions can be readily controlled. The interac- tion between the ambient condition and the platen temperature can be seen in the compara- tive results given in Figure 4. At the ambient condition of 40°C/93%RH a platen temperature of 38.5°C now causes significant condensation, CONDENSATION TESTING—A NEW APPROACH Figure 2: PCB temperature profile during platen thermal cycling. Figure 3: SIR response as the platen was cycled to cooler temperatures. Figure 4: SIR response at 40oC/93%RH under condensing conditions.